Determination of Flow Stress and Coefficient of Friction for Extruded Anisotropic Materials under Cold Forming Conditions

The work material in metal working operations always shows some kind of anisotropy. In order to simplify the theoretical analysis, especially considering bulk deformation processes, anisotropy is usually neglected and the material is assumed to be isotropic. On the other hand, the analysis that considered the influence of anisotropy seldom incorporates the influence of friction. For predicting the material flow during plastic deformation and for predicting the final material properties of the product, adequate descriptions of both flow stress curves and coefficients of friction have to be developed. In the present work a number of experimental methods for determining the anisotropy have been utilized and compared: Yield loci, strain ratios (R-values) and establishing flow stress-curves in different directions. The results show that the yield loci measurements are weak in predicting anisotropy when the material strain hardening is different in different directions. It is concluded that also the strain ration (R-value) measurements are unreliable for describing anisotropy. The most trustable and useful results were found from multi-direction determinations of the flow stresses. Three typical cases of ring upsetting conditions were analyzed by theory (3DFEM) and experiments: 1) An anisotropic ring, oriented 90 to the axis of rotational symmetrical anisotropy. The friction coefficient was the same in all directions 2) An isotropic ring. The friction coefficient was different in different directions 3) An anisotropic ring oriented 0 to the axis of rotational symmetrical anisotropy. The friction coefficient was the same in all directions The cases 1) and 2) reveal that the influence of anisotropy on the ring deformation is quite similar to that obtained by changing the frictional condition. The case 3) exposes that if the material flow caused by anisotropy is incorrectly referred to friction, the possible error of the friction coefficient can be as high as 80% for a pronounced anisotropic material. A modified two-specimen method (MTSM) has been established according to an inverse method. Experiments were carried as cylinder upsetting. Here both ordinary cylinders were used as well as so-called Rastegaev specimen. Also plane strain compression tests were utilized. The results show that MTSM is able to evaluate the validity of a selected mathematical model when both the friction coefficient and the flow stress are unknown for a certain process. MTSM can also be used to estimate the friction coefficient and flow stress provided that the selected mathematical model is adequate.